Accurate, rapid measurement of serum gentamicin levels is critical for optimal patient care because of the low toxic-therapeutic ratio of gentamicin and the consequent need to make dosage adjustments quickly. In this study, a bio-assay, fluorescent immunoassay, an enzyme immunoassay, a radio-immunoassay, and a latex agglutination inhibition test were compared for their utility in measuring serum gentamicin levels in a clinical setting.